Voice coil motors are widely used in computer disk drive systems to position a read head over a storage medium. A conventional voice coil motor includes a single coil connected between two pairs of driving transistors in an H-bridge circuit configuration. The coil is situated inside a permanent magnet and is connected to the driving transistors by a plurality of flex cables. The coil is free to move inside the magnet subject to a minimal amount of damping and friction from the flex cables. The coil is accelerated in a forward direction by driving current through the coil in a forward direction, and the coil accelerates according to the amount of current driven through it. The coil will maintain momentum in the forward direction until a current is driven through the coil in a reverse direction resulting in a negative acceleration. The coil will slow and stop according to the amount of current driven through it in the reverse direction, and a further application of current will accelerate the coil in the reverse direction.
The driving transistors in the H-bridge circuit are selectively energized to drive current into the coil to accelerate the coil. An arm is attached to the coil, and a read head is fixed to an end of the arm and is moved backwards and forwards in a linear path by the coil. The storage medium is typically a rotating disk with a magnetic surface having a plurality of concentric tracks in which digital data is stored. The read head is positioned by the coil over a track to read data in the track or to write data to the track. When the read head is not being used to access data on the disk, it is held in a stationary rest position over an area of the disk which is separated from the tracks.
Each end of the coil is connected between a high side driving transistor and a low side driving transistor in the H-bridge circuit. Typically, each of the driving transistors in a voice coil motor is an N channel double-diffused metal-oxide semiconductor (DMOS) transistor having a gate, a drain and a source. The driving transistor is rendered conductive, or is switched on, when a gate voltage applied to its gate exceeds a source voltage at the source of the driving transistor by a threshold voltage. As the driving transistor is switched on, a current begins to flow through the transistor from the drain to the source. Typically, the gate voltage is raised to two or three times the level of the threshold voltage above the source voltage. The driving transistor presents a resistance to the source-drain current which reduces as the gate voltage increases. When the gate voltage reaches a maximum, the driving transistor is in a saturated state and presents very little resistance to the source-drain. Conversely, the driving transistor is rendered non-conductive, or is switched off, by reducing the gate voltage until the resistance increases sufficiently to stop the current between the drain and the source. The coil is connected at each end between a high side driving transistor and a low side driving transistor.
In the H-bridge circuit current is directed through the coil by switching on a high side driving transistor on one side of the coil and a low side driving transistor on the other side of the coil to connect the coil between a voltage source and a ground. Current travels from the voltage source, through the high side driving transistor, the coil, the low side driving transistor, and then to the ground. At each end of the coil the voltage rises as the high side and low side driving transistors are switched on. Conversely, the voltage at either end of the coil falls to the ground after the driving transistors have been switched off. Any change in the voltage in the coil will include transient voltage spikes caused by a reaction of the coil against the change. The voltage at each end of the coil changes at a rate, called a slew rate, which is related to the rate at which the adjacent driving transistor is switched on or off by its gate voltage. The slew rate increases with an increase in the rate of change of the gate voltage of the adjacent driving transistor, and decreases with a reduction in the rate of change of the gate voltage.
The coil may be driven by a linear current or a pulse width modulated (PWM) current. A linear current is a constant current driven through the coil by switching on each appropriate driving transistor with a linear gate voltage signal so that the driving transistors provide a substantial resistance to the source-drain current. The linear gate voltage signals are used to regulate the current in the coil by increasing or decreasing the resistance of the driving transistors. The coil accelerates forward according to the amount of linear current driven into it in a forward direction. The coil is stopped or accelerated in a reverse direction according to the amount of linear current driven through it in the reverse direction.
The PWM current is generated in the coil by applying a PWM gate voltage signal to the gates of the appropriate driving transistors. The driving transistors are switched on and off by the PWM gate voltage signal. The PWM gate voltage signal is a series of voltage pulses in which each voltage pulse raises the gate voltage rapidly to a high voltage level which is held for a first selected time period. At the end of the first selected time period the gate voltage declines rapidly to a low voltage level that is held for a second selected time period. The ratio of the first selected time period to the total time period made up of the first and second selected time periods is known as the duty cycle of the PWM gate voltage signal. The PWM gate voltage signal switches on the driving transistors with a gate voltage sufficient to put the driving transistors in a saturated state such that they present a minimal resistance to current.
The PWM current may be induced in the coil in the forward direction by applying a PWM gate voltage signal to the appropriate high side and low side driving transistors. The PWM current will fluctuate or produce a ripple around an average value of current in the forward direction, the ripple resulting from the pulses in the PWM gate voltage signal. The average value of the forward PWM current in the coil is proportional to the duty cycle of the PWM gate voltage signal. The PWM current may be induced in the coil in the reverse direction by applying the PWM gate voltage signal to an opposing pair of high side and low side driving transistors. The coil is accelerated in a forward direction according to an average forward PWM current in the coil which is determined by the duty cycle of the PWM gate voltage signal. The coil is slowed or accelerated in a reverse direction according to an average reverse PWM current in the coil which is determined by the duty cycle of the PWM gate voltage signal.
The coil may be driven with a PWM gate voltage signal during a portion of its movement and by a linear gate voltage signal during a subsequent motion. The linear gate voltage signal may be used to precisely control the motion of the coil, but a drawback of the use of a linear gate voltage signal is that a significant amount of power is dissipated by the resistance of the driving transistors. An advantage of a PWM gate voltage signal is that the driving transistors dissipate much less power when driving the PWM current through the coil. However, the PWM gate voltage signal generates a significant amount of electromagnetic interference (EMI), or noise, which may interfere with a transmission of data in a read/write channel linking the read head with a disk drive system. The noise is generated by abrupt changes in current in the coil as well as fast voltage transients. The PWM gate voltage signal pulses have sharp edges and steep inclines which drive the coil and supply wiring as an antenna to generate the noise. One way of reducing the noise generated by driving a PWM current in the coil is to reduce the slew rate at each end of the coil because the current in the coil is directly related to the voltage in the coil. A reduction in the rate of change of voltage at each end of the coil reduces the rate of change of current in the coil. Reducing the slew rate also increases the amount of power dissipated by the driving transistors.
As described above, the coil in the voice coil motor moves the read head across the rotating disk to read data from or write data to an appropriate track on the disk. When the read head is not accessing data on the disk, it is parked in a rest position a substantial distance from the tracks. The movement of the read head takes place in two phases, a seek phase and a track and follow phase. In the seek phase, the read head is moved rapidly over a substantial portion of the disk from the rest position (or a previous track) to the neighborhood of a selected track. A substantial amount of current is applied to accelerate the read head toward the selected track. After the read head has traveled for a predetermined distance, a substantial current is applied in the reverse direction to slow the coil until the read head is positioned over the track. In the track and follow phase, a small amount of current is applied to move the coil to position and maintain the read head over the selected track. The tracks can be slightly eccentric around the center of the rotating disk and the read head must be moved to follow them.
In some recent disk drive systems, a PWM current is applied to accelerate and decelerate the coil in the seek phase to minimize power dissipation, while a linear current is applied to the coil in the track and follow phase. Because only a small amount of current through the coil is needed in the track and follow phase, a linear current can be used to drive the coil without significant power dissipation.
The position of the read head is determined by a control logic unit in the disk drive system from two sources of information. The read head itself reads position data from the rotating disk as it travels over the disk to the selected track and transmits this data to the control logic unit. In addition, the control logic unit retains in a memory preassigned velocity profiles to achieve a desired track position. The control logic unit senses the track position and the amount of current in the coil to determine the acceleration of the coil, and then compares this information with the velocity profiles stored in the memory to optimize the position of the coil.
The noise generated by the disk drive system is quantified by a signal-to-noise ratio (SNR) which is a ratio of the average power of a data signal being transmitted by the read head to the average power of the noise. When the read head is transmitting data to or from the disk in the track and follow phase, the SNR must be maximized to ensure that the data is not lost in the transmission. However, when the read head is reading position data from the rotating disk in the seek phase, a lower SNR is tolerable because missed data points may be interpolated from surrounding data points. In addition, the position of the read head is also determined from the current in the coil. Therefore, in modern disk drive systems the coil may be moved with a PWM current in the seek phase and with a linear current in the track and follow phase. However, the provision of both a PWM current and a linear current to drive the coil requires two distinct circuit loops in the disk drive system which is expensive.